Chen Ning Yang

Chen Ning Yang
(1922)

Chinese-born American theoretical physicist whose research with Tsung-Dao
Lee showed that parity--the symmetry between physical phenomena occurring
in right-handed and left-handed coordinate systems--is violated when
certain elementary particles decay. Until this discovery it had been
assumed by physicists that parity symmetry is as universal a law as
the conservation of energy or electric charge. This and other studies
in particle physics earned Yang and Lee the Nobel Prize for Physics
for 1957.

Life.
Yang's father, Yang Ko-chuen (also known as Yang Wu-chih), was a professor
of mathematics at Tsinghua University, near Peking. While still young,
Yang read the autobiography of Benjamin Franklin and adopted "Franklin"
as his first name. After graduation from the Southwest Associated University,
in K'unming, he took his B.Sc. in 1942 and his M.S. in 1944. On a fellowship,
he studied in the United States, enrolling at the University of Chicago
in 1946. He took his Ph.D. in nuclear physics with Edward Teller and
then remained in Chicago for a year as an assistant to Enrico Fermi,
the physicist who was probably the most influential in Yang's scientific
development. Lee had also come to Chicago on a fellowship, and the two
men began the collaboration that led eventually to their Nobel Prize
work on parity. In 1949 Yang went to the Institute for Advanced Study
in Princeton, N.J., and became a professor there in 1955. He became
a U.S. citizen in 1964.

Work.
Almost from his earliest days as a physicist, Yang had made significant
contributions to the theory of the weak interactions--the forces long
thought to cause elementary particles to disintegrate. (The strong forces
that hold nuclei together and the electromagnetic forces that are responsible
for chemical reactions are parity-conserving. Since these are the dominant
forces in most physical processes, parity conservation appeared to be
a valid physical law, and few physicists before 1955 questioned it.)
By 1953 it was recognized that there was a fundamental paradox in this
field since one of the newly discovered mesons--the so-called K meson--seemed
to exhibit decay modes into configurations of differing parity. Since
it was believed that parity had to be conserved, this led to a severe
paradox.
After exploring every conceivable alternative, Lee and Yang were forced
to examine the experimental foundations of parity conservation itself.
They discovered, in early 1956, that, contrary to what had been assumed,
there was no experimental evidence against parity nonconservation in
the weak interactions. The experiments that had been done, it turned
out, simply had no bearing on the question. They suggested a set of
experiments that would settle the matter, and, when these were carried
out by several groups over the next year, large parity-violating effects
were discovered. In addition, the experiments also showed that the symmetry
between particle and antiparticle, known as charge conjugation symmetry,
is also broken by the weak decays. (See also CP violation.)

In addition to his work on weak interactions, Yang, in collaboration
with Lee and others, carried out important work in statistical mechanics--the
study of systems with large numbers of particles--and later investigated
the nature of elementary particle reactions at extremely high energies.
From 1965 Yang was Albert Einstein professor at the Institute of Science,
State University of New York at Stony Brook, Long Island. During the
1970s he was a member of the board of Rockefeller University and the
American Association for the Advancement of Science and, from 1978,
of the Salk Institute for Biological Studies, San Diego. He was also
on the board of Ben-Gurion University, Beersheba, Israel. He received
the Einstein Award in 1957 and the Rumford Prize in 1980; in 1986 he
received the Liberty Award and the National Medal of Science.

BIBLIOGRAPHY.
Jeremy Bernstein, A Comprehensible World (1967), is a collection of
essays on modern science containing a profile of Lee and Yang first
published in The New Yorker magazine; H.A. Boorse and L. Motz (eds.),
The World of the Atom, vol. 2 (1966), is a collection of popular and
semipopular papers on modern physics containing both biographical sketches
of Lee and Yang and some of their popular essays. Yang's Selected Papers
1945-80 with Commentary (1983) includes personal memories, letters,
and recollections of developments in theoretical physics since World
War II.